Power-driven extension ladders



April 19, 1960 R. o. BALOGH 2,933,150

POWER-DRIVEN EXTENSION LADDERS Filed March 15, 1954 3 Sheets-Sheet 2 I I I I I l l A 4/41 1 FIG. 2..

INVENTOR; Rm 0. BALOGH April 1960 R. o. BALOGH 7 2,933,150

POWER-DRIVEN EXTENSION LADDERS Fiied March 15, 1954 3 Sheets-Sheet 5 FIG. IO.

INVENTOR. Rm 0. BALOGH United States Patent POWER-DRIVEN EXTENSION LADDERS Roy A. Balogh, Clayton, Mo., assignor to McCabe- Powers Body Company, a corporation of Missouri Application March 15, 1954, Serial No. 416,088

15 Claims. (Cl. 182.208)

This invention relates in general to certain new and useful improvements in aerial ladders and, more particularly, to a power-driven extension ladder.

Public utility companies, municipalities, and similar public and private corporations employ motor vehicles equipped with aerial laddersof various types for servicing overhead electrical lines, light poles, telephone lines, electrical display signs, and even tree-trimming operations. Because of the wide variety of different operations for which aerial ladders are ordinarily used, it is necessary that such ladders be capable of adjustment to various heights and lengths quickly and economically. However, in order to employ trucks, of conventional chassis-length and still be able to achieve a satisfactory maximum height, it is desirable to employ a ladder of the extension type comprising two or more telescoping sections. The various types of vehicular-mounted-aerial extension ladders which are presently available are cumbersome and .difiicult to manipulate,- requiring a relatively large amount of physical force for extending .the several sections. Furthermore, existing types of aerial extension ladders possess an extensive amount of play between the various sections, so that, when such ladders are in fully extended position, a workman may feel somewhat insecure when aloft.

It is, therefore, the primary object of the present vention to provide an aerial extension ladder including two or more telescoping sections which may be shifted hl relation to each other by power-driven means efiiciently, quickly, and economically. i

It is another object of the present invention to provide an aerial extension ladder having a plurality of telescoping sections which are readily shiftable with respect to each other with a minimum of force and power and, at the same time, are firmly interconnected so that the sections will not wobble or shake with respect to each other in either fully telescoped position, fully extended position, or any intermediate operating position.

It is an additional object of the present invention to provide an aerial extension ladder comprising a plurality of telescoping sections and having power-driven means for shifting the sections with respect to each other.

It is also an object of the present invention to provide a power-driven aerial extension ladder in which the driving means is uniquely compact and directly coupled to the load and employs a continuous or endless cable drive trained around driving pulleys which do not permit sidewise translation of the cable as it travels therearound.

With the above and other objects in view, my invention resides in the novel features of form, construction, arrangement, and combination of parts presently described and pointed out in the claims.

In the accompanying drawings (three Sheets);-

Figure 1 is a rear end elevational view of a truck equipped with an aerial extension ladder constructed in accordance with and embodying the present invention;

Figures 2, 3 and 4 are transverse sectional views taken 2,933,15fi Patented Apr. 1a, 1950 2 a e ts li s 1 3 n 1- Figure 1;-

\ Figures 5 and 6 are fragmentany sectional views taken along lines 55 and 66 respectively, of Figure 2;

'Figure 7 is a fragmentary sectional viewtaken along line 7..7 of Figure 3;

Figures 8 and 9 are fragmentary sectional views taken along lines 3-8 and 9-,9, respectively, of Figure 4;

Figures 10 and 11 are fragmentary sectional views taken along lines Ill-10 and 11-41, respectively, of Figure 8;

Figures 12 and 13 are diagrammatic views of the cable pulley respectively showing the positions of the cable during ladder-raising and ladder-lowering operation; and

Figure 14 is a fragmentary sectional view taken along line l4 14 of Figure 1.

Referring now in more detail and by reference characters to the drawings, which illustrate a preferred embodiment of the present invention, A designates a conventional utility truck having a rearwardly extending floor l. Rigidly mounted upon the floor 1 is a metal base-plate .2 and welded upon the upper face thereof is a vertical tubular post 3 stabilized by four angular bracebars 4. The base-plate 2, post 3, and brace-bars 4- are preferably welded together as an integral unit for convient mounting upon any standard truck or vehicle.

Rotatively mounted upon the upper end of the post 3, preferably in the manner more fully described in my co pending application Serial No. 331,241,. filed January 1.4, 1953, now Patent No. 2,777,737, is a tubular post sleeve 5 extending downwardly and concentrically around the post 3 and terminating just above the upper end of the bracebars 4. The tubular post sleeve 5 is furthermore provided at its upper end with a pair of axially aligned diametrally projecting pintles 6 and journaled thereon at its lower end is a main ladder-section 7 consisting of two spaced parallel side rails 8, 9, formed of tubular metal channels interiorly provided with reinforcing rails 10, 11, formed preferably of wood or similar dielectric material and projecting outwardly beyond the upper end of the Tails 8, 9, respectively. The rails 8, 9, and the associated reinforcing rails 10, 11, are transverse- 1y connected by a plurality uniformly spaced tubular respectively, of

rungs 12. Also extending between the side rails 8, 9, in

parallel relation to the lowermost rung 12 and spaced downwardly therefrom by a space equal to the uniform spacing between the several rungs 12, is a tubular rung 13 welded or otherwise rigidly fastened at its ends to mounting brackets 14, each of which integrally includes a bight 15, short clearance-legs 16, and attachment flanges 17, the latter being adapted for receiving bolts 18 which extend through the rails 8, 9, respectively, as best seen in Figure 5.

Loosely and rotatively mounted within, and extending axially through, the run 13 is a drive-shaft 19 journaled at its ends in conventional bearings b, b, respectivelyset into the side rails 8, 9. One end of shaft 19 projects through the side rail 8 and auxiliary rail 10 and is drivingly connected to a rotary oil-motor 20. The details of the oil-motor 20 and the hydraulic system employed in connection therewith form the subject-matter of a related patent application filed contemporaneously herewith under Serial No. 416,339.

Keyed to each end of the shaft 19 for operation within the clearance space afforded by'the brackets 14 are identical driving pulleys 21 integrally including annular flanges 22 and a concavely indented rim-face 23 with its smallest diametral size lying in substantially the central plane of the pulley. Similar pulleys 21' are journaled, upon the uppermost rung 12 in lengthwise alignment with the pulleys 2-1 by means of suitable. anti-friction bushings 2 h s a e d in Pla by nt ona e r se Trained around each pair of longitudinally aligned pulleys 21, 21, is a continuous or endless driving cable 26. It will be noted that each of the driving cables 26 is wrapped twice around'the rim-face 23 of each of the pulleys 21, 21', that is to say, each of the rim-faces 23 is encircled by one and a half turns of the cable 26.

Rigidly bolted in longitudinally spaced relation along the upper end of the reinforcing rails 10, 11, are L- shaped bracket-plates 27 integrally provided with upstanding posts 28 having tr'ansversely extending tubular sleeves 29 for supporting roller-spindles 30, and journaled thereon are rubber rollers 31. Shiftably mounted between the rollers and the rungs 12 is an extension laddersection 32 including spaced parallel side-rails 33, 34, and rungs 35. As will be seen by reference to Figures 3 and 4, the side rails 33, 34, bear slidably along their under faces against the rungs 12 and along their upper faces bear against the rollers 31. The ladder-section 32 is substantially narrower than the main ladder-section 7 and, therefore, fits telescopically between the rails 8, 9,

between the cables 26. Each of the side rails 33, 34, is furthermore provided at a suitable location, preferably near its lower end, with a pair of parallel clamp-plans 36, 37, having complementarily formed lips 38, 39, for gripping engagement with the adjacent cable 26, being drawn tightly into cable-engaging position by means of a bolt 40. It should be noted in this connection that the outwardly presented face of each plate 37 is dished inwardly, as shown in Figure 10, and the plate 36 is narrowed to fit thereinto, so that the engaged portion of the cable 26 is deflected slightly to insure positive non-slipping retention.

Also bolted or otherwise rigidly secured to the reinforcing rails 10, 11, in proximate alignment to the bracket-plates 27, are auxiliary L-shaped bracket-plates 41' having upwardly and angularly inwardly extending leg-portions 42 terminating in short substantially rigid lateral flanges 43 which overlie the upwardly presented edge-faces of the side-rails 33, 34, of the extension laddersection 32. As will be seen by reference to Figure 14, there is a small amount of clearance between the under face of the flanges 43 and the side-rails 33, 34, which is slightly smaller than the amount of dimensional compression in the rubber rollers 31. Consequently, when the extension ladder-section 32 is being shifted upwardly or downwardly, it is held in place by the rubber rollers 31. When, however, the extension ladder-section 32is stopped and locked in any operative position, and is placed under load through the weight of a workman who has climbed to a working position, the weight of the workman will compress the rubber rollers 31 slightly, so that the load of the extension ladder-section 32 will be transferred to the rigid auxiliary brackets41. Consequently, under load the two ladder-sections 7, 32, will be held togetherin a relatively rigid manner'rand the workman will feel safe and secure when aloft.

As more fully set forth in the above-mentioned copending application, the oil-motor 20, by proper manipulation of valves (not herein shown), can be made to rotate in either direction, transmitting corresponding rotation to the shaft 19 and pulleys 21, which, in turn, drive the cables 26 to raise or lower the ladder-section 32 as the case may be. The ladder-section 32 is also provided with the usual typeof gravity-actuated rungengagin'g latch-pawl 44 by which it is held in any selected position of extension, but since this latch p'awl 44 is en- 4 cables 26 can be kept taut, the ladder-section 32 will track up and down smoothly and without side-sway or longitudinal misalignment. Finally, the retentive engagement of the rollers 31 hold the ladder-section 32 against any substantial or significant amount of lateral movement of so-called wobble, so that the user, when aloft, will 7 feel firmly supported and secure.

- It should also be noted that the application of power directly to the axle 19, which extends through the rung 13, assures efficient smooth operation and easily 'controlled'extension and retraction of the ladder-section 32.

It should be'understood that changes and modifications in the form, construction, arrangement, and combination of the several parts of the power-driven extension ladder may be made and substituted for those herein shown and described without departing from the nature and principle of my invention.

Having thus described my invention, what I claim and desire to secure by Letters Patent is 1. An aerial ladder comprising a main ladder-section, a second ladder-section slidably mounted on the main ladder-section for telescopic movement with respect thereto, resilient means operatively mounted on the main ladder-section and being disposed against the second ladder-s'ection, stress-taking means rigidly connected to the mairr ladder-section and normally overlying the second ladder-section, said stress-taking means being engageable with said second ladder-section when the resilient means "distend under the weight of a workman standing on the second ladder-section, a pair of pulleys mounted in longitudinally spaced relation along the main ladder-section, an endless cable extending longitudinally of the main laddel -section, said cable being tautly trained and snubbed around the pulleys, an oil motor carried by the main ladder-section and operatively connected to the endless cable, and a clamp-plate connecting the second ladder-section to the cable.

2. An aerial ladder comprising a main ladder-section having spaced parallel side rails, a plurality of spaced parallel rungs secured at their ends to the side rails, a hollow tubular rung adjacent to the lower end of the main laddersection, the ends of said hollow tubular rung being open and secured to at least one of the side rails by a U-shaped bracket which holds the end of said hollow rung in spaced relation to such side rail, a second ladder-section slidably mounted on the main ladder-section for telescopic movement with respect thereto, a first pulley rotatably mounted on one of the upper rungs, an endless flexible member extending longitudinally of the main ladder-section and trained around the first pulley, a rotary hydraulic motor carried by the main ladder-section and having a rotating -driveshaft extending through the hollow rung, a second pulley fixed upon the drive shaft and operably located in tirely conventional and not a part of the present in'vention, it is not shown or described in detail.

By reason of the concavely indented form of the rimfaces 23, thecables 26 will track smoothly and continuously around the pulleys 21, 21', and will not feed laterally along the rim-faces 23 in the direction of the longitudinal axis of rotation thereof. Consequently, the cables 26 will not tend either to run off or to bind with respect to the pulleys 21,21. Furthermorejsince the the space afforded by the U-shaped bracket, said flexible member also being trained around saidsecond pulley and drivingly engaged therewith, and means for connecting the endless. flexible member to the second ladder-section wherebythe latter may be shifted with respect to the main ladder section.

3. An aerial ladder comprising a main ladder-section having spaced parallel side rails, a plurality of spaced parallel rungs secured at their ends to the side rails, a hollow tubular rung adjacent to the lower end of the main ladder-section, the ends of said hollow tubular rung being open and secured toat least one of the side rails by a U-shaped bracket which holds the end of'said hollow rung in spaced relation to such side rail, a second laddersection slidably mounted on the mainladder-section for telescopic movement with respect thereto, a rotary drive shaft operatively mounted in and extending through the hollow'rung, a first pulley rotatably and concentrically mounted on one of the upper rungs in peripheral alignmentwith the space afforded by the U-shaped bracket, a second pulley fixed upon the drive'shaft and operably located in the space afiordedby the U-shaped bracket,

e d es sable x ndin lo tud na of the mai ladder-section, said cable being-'tautly trained andsnubbed around the pulleys, a rotary hydraulic motor carried by the main ladder-section and operatively connected to the drive shaft, and a clamp-plate connecting the second ladder-section to the cable.

4. An aerial ladder comprising a main ladder-section having apair of spaced side-rails and aplurality of spaced parallel rungs extending transversely between the siderails, one of said rungs being tubular and being provided at its ends with yoke-like brackets adapted for rigidly supporting the ends of said tubularrung in inwardly spaced relation to the inner faces of the side-rails, a shaft extending operatively through the tubular rung and projecting at one end through one of the siderails, a motor ,drivingly connected to the projecting end of the shaft,

driving pulleys mounted onthe shaft in thespaces between the ends of the tubular rungand the side-rails, auxiliary pulleys rotatably nounted adjacent the other end of the main ladder-section, each of said driving pulleys being paired and sidewise aligned with an auxiliary pulley, a second ladder-section slidably mounted on the main ladtier-section for telescopic movementwithrespect thereto,

resilient means operatively mounted on the main ladder- ;section and being disposed against the second ladder-section, stress-taking means rigidly connected to the main ladder-section and normally overlying thesecond ladder- ,section, said stress-taking means being engageable with said second'ladder-section when the resilient means distend under" the weight of a workman standing on the secondlad'der-section, endless cables trained around said paired pulleys, and means for fastening the secondjladdersection to selected portions of the endless cables whereby the second ladder-section may be shiftedlongitudinally with respect to the mainladdensection.

5. An aerial ladder comprising a main ladder-section havinga pair of spaced side-rails and a plurality of spaced parallel rungs extending transversely between the sidetrails, one of said rungs being tubular and being provided atits ends with yoke-like brackets adapted for rigidly supporting the ends of said tubular rung in inwardly spaced relation to the inner faces of the side-rails, a shaft extending operatively through the tubular rung and projecting at one end through one of the side-rails, a motor drivingly connected to the projecting end of the shaft, driving pulleys mounted on the shaft in the spacesbetween the ends of the tubular rung and the side-rails, auxiliary pulleys rotatably mounted adjacent the other end of the main ladder-section, each of said driving pulleys being paired and sidewise aligned with anauxiliary pulley, a secondladder section slidably mounted on the main ladder-section for telescopic movement with respect thereto, endless cables trained a plurality of turnsaround said paired pulleys, and means for fastening the second ladder-sectionto selected portions of the endless cables whereby the second ladder-section may betshifted longitudinally with respect to the main ladder-section.

. ends of said tubular rung in inwardly spaced relation to the inner faces of the side-rails, a shaft extending operatively through thetubular rung and projecting at one end through one of the side-rails, a motor drivingly connected to the projecting end of the shaft, driving pulleys mounted on the shaft in the spaces between the ends of the tubular rung and the side-rails, auxiliary pulleys rotatably 'mounted adjacent the other end of the mainladder-section, each, of said driving pulleys being paired andsidewise aligned withan auxiliary pulley, a second laddersection slidably mounted on the mainladder-sectionfor telescopicmovement with respect thereto, endless cables portions of the endless cables whereby the second laddersection may be shifted longitudinally with respect to the main ladder-section.

7. An aerial ladder comprising a main ladder-section having a pair of spaced side-rails and a plurality of spaced parallel rungs extending tranversely between the side-rails, one of said rungs being tubular and being provided at its ends with yoke-like brackets adapted for rigidly supporting the ends of said tubular rung in inwardly spaced relation to the inner faces of the side-rails, a shaft extending operatively through the tubular rung and projecting atone end through one of the side-rails, a motor mounted rigidly on the main ladder-section and drivingly connected to the projecting end of the shaft, driving pulleys mounted on the shaft in the spaces between the ends of the tubular rung and the side-rails, auxiliary pulleys rotatably mounted adjacent the other end of the main ladder-section, each of said driving pulleys being paired and sidewise aligned with an auxiliary pulley, a second ladder-section slidably mounted on the main ladder-section for telescopic movement with respect thereto, resilient means operatively mounted on the main ladder-section and being disposed against the second ladder-section, stress-taking means rigidly connected-to the main laddersection and normally overlying the second ladder-section, said stress-taking means being engageable with said second ladder-section when the resilient means distend under the weight of a workman standing on the second laddersection, endless cables trained around said paired pulleys, and means for fastening the second ladder-section to selected portions of the endless cables whereby the second ladder-section may be shifted longitudinally with respect to the main ladder-section, each of said pulleys including axially spaced annular flanges and a diametrally reduced concave rim-face extending axially therebetween.

8. An aerial ladder comprising a main ladder-section having a pair of spaced side-rails and a plurality of spaced parallel rungs extending transversely between the siderails, one of said rungs being tubular and being provided at its ends with yoke-like brackets adapted for rigidly supporting the ends of said tubular rung in inwardly spaced relation to the inner faces of the side-raiis, a shaft extending operatively through the tubular rung and projecting at one end through one of the side-rails, a motor mounted rigidly on the main ladder-section and drivingly connected to the projecting end of the shaft, driving pulleys mounted on the shaft in the spaces between the ends of the tubular rung and the side-rails, auxiliary pulleys rotatably mounted adjacent the other end of the main laddersection, each of said driving pulleys being paired and sidewise aligned with an auxiliary pulley, a second laddersection slidably mounted on the main ladder-sectionfor telescopic movement with respect thereto, endless cables trained around said paired pulleys, and means for fastening the second ladder-section to selected portions of the endless cables whereby the second laddensection may be shifted longitudinally with respect to the main ladder-section, each of said pulleys including axially spaced annular flanges and a diametrally reduced rim-face extending axially therebetween and having the shape of two axially aligned, endwise converging, frusto-conical sections, the axial Width of said rim 'face between said flanges being substantially greater than twice the thickness of the cable so that the cable will shift axially across the rim face as the direction of longitudinal motion of the cable is changed but will not translate axially as long as cablemotion is continuous in one direction or the other.

9. An aerial ladder comprising a main ladder-section having a pair of spaced side-rails and a plurality of spaced supporting the ends of said tubular rung in inwardly spaced relation to the inner faces of the side-rails, a shaft connected to the projecting end of the shaft, driving pulleys mounted on the shaft in the spaces between the ends of the tubular rung and the side-rails, auxiliary pulleys V rot'atably mounted adjacent the other end of the main ladder-section, each of said driving pulleys being paired and sidewise aligned with an auxiliary pulley, a second ladder-section slidably mounted on the main ladder-section for. telescopic movement with respect thereto, endless cables trained a plurality .of side-by-side turns around said paired pulleys, and means for fastening the second ladder-section to selected portions of the endless cables whereby the second ladder-section may be shifted longitudinallywith respect to the main ladder-section, each of said pulleysincluding axially spaced annular flanges and a diametrally reduced rim-face extending axially therebetween and having the shape of two axially aligned, endwise converging, identical frusto-conical sections.

10. An aerial ladder comprising a main ladder-section having a pair of spaced side-rails and a plurality of spaced parallel rungs extending transversely between the siderails, one of said rungs being tubular and being provided at its ends with yoke-like brackets adapted for rigidly supporting the ends of said tubular rung in inwardly spaced relation to the inner faces of the side-rails, a shaft extending operatively through the tubular rung and project- .ing at one end through one of the side-rails, a motor drivingly connected to the projecting end of the shaft,

driving pulleys mounted on the shaft in the spaces between the ends of the tubular rung and the side-rails,

.auxiliary pulleys rotatably mounted adjacent the other end of the main ladder-section, each of said driving pulleys being paired and sidewise aligned with an auxiliary pulley, a second ladder-section slidably mounted on the main ladder-section for telescopic movement with respect thereto, resilient rollers rotatably mounted on the main ladder-section and having their peripheries disposed against the second ladder-section, stress-taking means rigidly connected to the main ladder-section and including flange means normally overlying the second laddersection, said flange means being engageable with said second ladder-section whenthe rollers distend under the weight of a workman standing on the second ladder-section, endless cables trained around said paired pulleys,

and means for fastening the second ladder-section to selected portions of the endless cables whereby the second ladder-section may be shifted longitudinally with respect to the main ladder-section.

11. An aerial ladder comprising a main ladder-section having a pair of spaced side-rails and a plurality of spaced parallel rungs extending transversely between the siderails, one of said rungs being tubbular and being provided at its ends with yoke-like brackets adapted for rigidly supporting the ends of said tubular rung in inwardly spaced relation to the inner faces of the side-rails, a shaft extending operatively through the tubular rung and projecting at one end through one of the side-rails, driving means operatively connected to the projecting end of the shaft, driving pulleys mounted on the shaft in the spaces between the ends of the tubular rung and'the side-rails, auxiliary pulleys rotatably mounted adjacent the other end of the main ladder-section, each of said driving pulleys being paired and sidewise aligned with an auxiliary pulley, a second ladder-section slidahly mounted on the main ladder-section for telescopic movement with respect thereto, resilent means operatively mounted on the main ladder-section and being disposed against the second ladder-section, stress-taking means rigidly connected to the main ladder-section and normally overlying the second ladder-section, said stress-taking means being engageable with said second ladder-section when the resilient means distend under the weight of a workman standing on the second ladder-section, endless cables trained around said paired pulleys, and means for fastening the second ladat'its ends with yoke-like brackets adapted for rigidly supporting the ends of said tubular rung in inwardly spaced relation to the inner faces'of the side-rails, a shaft extending operatively through the tubular rung, driving means operatively connected to the shaft, driving pulleys mounted on the shaft in the spaces between the ends of the tubular rung and the side-rails, auxiliary pulleys rotatably mounted adjacent the other end of the main ladder-section, each of the driving pulleys being paired and sidewise aligned with an auxiliary pulley, a second laddersection'slidably mounted on the main ladder-section for telescopic movement with respect thereto, endless cables trained a plurality of turns and snubbed around said paired pulleys, 'said'second ladder-section being narrower than the transverse distance between the cables, and means for connecting the second ladder-section to the cables so that it is held between the cables and thereby having a pair of spaced side-rails and a plurality of spaced parellel rungs extending transversely between the siderails, one of said rungs being tubular and being provided 7 at its ends with yoke-like brackets adapted for rigidly supporting the ends of said tubular rung in inwardly spaced relation to the inner faces of the side-rails, a shaft extending operatively through the tubular rung, driving means operatively connected to the shaft, driving pulleys mountedon the shaft in the spaces between the ends of the tubular rung and the side-rails, auxiliary pulleys rotatably mounted adjacent the-other end of the main ladder-section, each of the driving pulleys being paired and sidewise aligned with an auxiliary pulley, a second laddersection slidably mounted on the main ladder-section for telescopic movement with respect thereto, endless cables trained tautly around said paired pulleys, said second ladder-section being narrower than the transverse distance between the cables, means for connecting the second ladder-section to the cables, andmeans for resiliently holding the second ladder-section slidably against the rungs of the main ladder-section so that it is held between the cables and thereby constrained against sidewise movement as it is shifted up and down.

14. An aerial ladder comprising a main ladder-section having a plurality of flat-faced rubber rollers mounted in outwardly spaced relation to one face thereof, a second ladder-section slidably mounted on the flat-faced rubber connected to the cable, means for operatively connecting the cable and the second ladder-section, and a rigid auxiliary bracket associatedjwith each of the rubber rollers, said brackets having fiat stress-taking flanges overlying the second ladder-section and spaced outwardly therefrom slightly beyond the plane of contact between the rubber roller and said second ladder-section, said flanges being adapted for engaging the second ladder-section when the rubber rollers compress under the weight of a workman standing on the second ladder-section.

15. An aerial ladder comprising a main ladder-section, a second ladder-section slidably mounted on the main ladder-section for telescopic movement with respect thereto, a pair of pulleys mounted in longitudinally spaced relation along the main ladder-section, an endless cable extending longitudinally of the main ladder-section, said driving means carried by the main ladder-section and operatively connected to the endless cable, clamping means connecting the second ladder-section to the cable, means for resiliently holding the second ladder-section slidably against the main ladder-section so that the second ladder-section is constrained against sidewise movement as it is shifted up and down, and stress take-up means mounted on said main ladder-section and being normally spaced from said second ladder-section, said stress takeup means being adapted to engage said second ladder-section when the resilient means distends under a substantial load.

References Cited in the file of this patent UNITED STATES PATENTS 10 Stewart Nov. 8, Frizell July 24, Cremar May 25, Bailey Dec. 20, Holmqvist July 1, Webster Aug. 29, Hayward Jan. 30, Painter Apr. 17, Setzer iuly 27, Hogan Ian. 16, Rose Aug. 14, Humpal et al. Oct. 6, Troche et a1. Jan. 26,

FOREIGN PATENTS Great Britain June 14,

Germany Sept. 18,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2 933, 150 April 19, 1960 Roy 0. Balogh I It is hereby certified that error appears in the above numbered patent requiring correct ion and that the said Letters Patent should read ascorrected below,

In the grant, line 1, and in the heading to the printed specification, line 3, name of inventor, for "Roy AB Balogh", each occurrence, read Roy 06 Balogh "q Signed and sealed this 27th day of September 1960a (SEAL) Attest:

KA H AXLIN ROBERT c. WATSON Attesting Officer Conmissioner' of Patents 

